1. Field of the Invention
The present invention relates to a driving force control apparatus configured to control driving forces to be applied to front left and right wheels and rear left and right wheels of a four-wheel drive vehicle.
2. Description of the Related Art
In an electric vehicle such as an electric automobile, left and right drive wheels are driven by respective electric motors or a common electric motor. The driving forces of the left and right drive wheels are controlled, so as to that the sum of those driving forces is equal to a driving force required by a driver, to take the same value, or to take values different from one another by optionally, independently controlling outputs of respective electric motors.
Respective drive wheels are in contact with a road surface through tires, and the driving force is transmitted to the road surface via the tires, with the result that energy is consumed by the tire treads of the respective wheels. Thus, it is possible to extend the cruising distance of the electric vehicle per battery charge by reducing the energy consumed by the tire treads of the respective drive wheels.
For example, in Japanese Patent Application Laid-open No. 2011-188561, there is disclosed an electric vehicle configured to control the driving forces of the left and right drive wheels such that a yaw moment produced by a difference between the driving forces of the left and right drive wheels takes an optimal value for a steady state circular turning of the vehicle determined by a steering angle. With the electric vehicle disclosed in Japanese Patent Application Laid-open No. 2011-188561, cornering resistances of the left and right drive wheels can be reduced, thereby being capable of reducing energy consumed when the tires of respective drive wheels slide in a lateral direction on the treads.
In the electric vehicle disclosed in Japanese Patent Application Laid-open No. 2011-188561, consideration is not given to a case where the tires of respective drive wheels slide in a longitudinal direction on the treads and a case where lateral forces of the tires change due to a load shift in the longitudinal direction caused when the vehicle accelerates or decelerates. Thus, it is only possible to reduce the energy that is consumed due to a lateral slide of the tires of respective drive wheels on the treads caused when the vehicle makes a turn at a constant vehicle speed. In other words, it is not possible to reduce energy that is consumed due to a longitudinal slide of the tires of respective drive wheels on the treads caused as in the case of the vehicle making a turn with acceleration or deceleration.
Further, in the electric vehicle disclosed in Japanese Patent Application Laid-open No. 2011-188561, consideration is not given to optimization of the driving forces of unsteered wheels. Thus, in the four-wheel drive vehicle in which driving forces are applied to the front left and right wheels and the rear left and right wheels, it is not possible to optimally control the driving forces of the four wheels so as to reduce energy consumed when the tires of respective drive wheels slide on the treads.